1. Field of the Invention
The present invention relates to method and device for detecting the end point of plasma process.
2. Description of the Related Art
The etching apparatus in which plasma is used has been variously incorporated into the course of manufacturing semiconductor devices, and substrates for liquid crystal display devices. It includes upper and lower electrodes arranged parallel to each other and it is intended to etch semiconductor wafers with active species in plasma while discharging between the upper and the lower electrode to make etching gas into plasma. When the semiconductor wafer is being etched, the progress of the etching process is observed and the end point of it is as correctly detected as possible to pattern the wafer as desired.
Conventionally, instrumental analysis methods such as mass spectrometry and spectroscopic analysis are used to detect the end point of etching process. Relatively simple and high sensitive one of them is the spectroscopic analysis. When the spectroscopic analysis is used, a specific active species is selected from radicals, ions and others of etching gas, its decomposed and reacted products and light strengths of the emission spectrum of this active species selected are measured. The active species selected depends upon the kind of etching gas in this case. When etching gas of the fluorocarbon series such as CF.sub.4 is used to etch silicon oxide film, spectrum (219 nm, 483.5 nm or others) emitted from the reacted product CO* is measured. When the fluorocarbon series etching gas such as CF.sub.4 is used to etch silicon nitride film, spectrum (674 nm or others) emitted from the reacted product N* is measured. The end point of etching process is decided by comparing changing values, which represent the light strength of the above-mentioned active species having a specific wavelength and primary and secondary differential ones of these light strength values, with a threshold previously set.
In the case of the conventional end point detecting methods, however, the active species whose spectrum is measured must be changed depending upon the kind of process used and also the kind of film etched. The threshold, therefore, must also be changed every active species used. Further, when films to be etched belong to the same kind but they are different in thickness, the etching condition under which one of them is etched must be changed from that for the other. This makes it necessary to change the threshold to meet the etching condition changed. In short, the threshold must be changed every etching condition employed, under different etching condition. It needs complexed calculations to set the threshold.